Shiny Pokémon were introduced in Pokémon GO on March 22nd 2017 as part of the special Water Festival event. Shiny Pokémon are Pokémon of a different colour to normal. During this event, it was quickly discovered that Magikarp have a chance of being Shiny. However, there are a few notes about Shiny Pokémon in this game.

First, a Shiny Pokémon is only for you. If two people capture the same Pokémon and it is Shiny for one, it will not be shiny for any other player.
In addition to this, a Shiny Pokémon will not appear as Shiny in the overworld. It will instead be normal and then appear as Shiny when you go to capture it.

If you go to transfer a Shiny Pokémon, you will get an additional prompt, however no other differences exist for Shiny Pokémon